The "Folks from Africa being held in the bottom of ships in Harsh conditions" didn't write or sing Amazing Grace as 1. They couldn't write or even speak English.

The song was written by a slave ship captain who got the crap scared out of him by a terrible storm off the coast of Ireland.

The song was written (based on that experience) years after he had left the slave-trading/seafaring business. It's all pretty well documented.

chas